This invention relates to a novel method of etching precisely-sized apertures into a continuous moving strip of metal, wherein the widths of the apertures may be smaller than the thickness of the strip. The etched product may be used to make shadow masks for color television picture tubes, as well as other precision-etched products.
A common type of color television picture tube comprises an evacuated glass envelope having a viewing window, a luminescent viewing screen supported on the inner surface of the viewing window, a shadow mask closely spaced from the viewing screen and an electron-gun mount assembly for generating one or more electron beams for selectively exciting the screen to luminescene. The shadow mask, which is a thin metal membrane having precisely-sized and spaced apertures therethrough, is used as a photographic master for making the screen, and then is used, during the operation of the tube, to aid in color selection on the screen by shadowing the electron beams. In both of these functions, it is important that the apertures therein follow closely in sizes and spacings with the mask specifications.
The mask is made by etching apertures into a strip of cold-rolled steel or other etchable metallic material thereby producing a flat mask, removing the flat mask from the strip, and then forming the flat mask to a desired shape. The strip is ordinarily about 0.15 mm (6 mils) thick, but may be as thin as 0.10 mm (4 mils) and as thick as 0.20 mm (8 mils). The apertures may be round or slit-shaped and may range in diameter or width from about 0.25 mm (10 mils) to less than the thickness of the strip. In order to etch such small-sized apertures, both major surfaces of the strip carry stencils thereon of etch-resistant material having openings therein that are registered with one another. The stencil openings in the obverse side or "O-side" of the strip are smaller and close to the sizes desired in the etched apertures. The stencil openings in the reverse side or "R-side" of the strip are larger than the sizes of the desired etched apertures. During the etching step, most of the etching occurs on the R-side, thereby permitting the etched apertures to be tapered and the cross sections of the apertures at their narrowest dimensions to be determined by the sizes of the stencil openings on the O-side.
With thin strips, further control during the etching step can be achieved by shielding the O-side from etchant during a portion of the etching step. U.S. Pat. No. 3,679,500 to N. Kubo et al. issued July 25, 1972 suggests briefly etching both sides of the strip, then coating the O-side with an etch-resistant material and then etching through only from the R-side. U.S. Pat. No. 2,750,524 to F. G. Braham issued June 12, 1956 suggests temporarily coating the O-side with etch-resistant material, then etching partially through from the R-side, then stripping off the temporary coating and then etching completely through from both sides. U.S. Pat. Nos. 3,971,682 issued July 27, 1976 and 4,013,498 issued Mar. 22, 1977, both to J. J. Frantzen et al., employ the method disclosed in Braham supra but substitute, for the temporary coating, a plastic sheet that temporarily lies upon the O-side during the initial portion of the etching step and then is removed. U.S. Pat. No. 4,142,437 to H. H. Bond et al. issued Nov. 7, 1978 discloses another variation of the method disclosed in Braham, supra.
The principle of shielding the O-side of the strip during a portion of the etching step can achieve the goal of producing precisely-etched apertures that may be smaller than the thickness of the strip. However, the prior methods for implementing this principle are difficult to work with and are not particularly cost effective. The novel method applies the same principle as is disclosed in Braham, but in a different way that is easier and less costly to implement than the prior methods mentioned above.